The growing popularity of electronic cigarettes has led to an increasingly fierce public health dispute—the great vape debate, if you will. On the one hand, there are doctors and advocates who insist that vaping is a relatively safe, appealing way for smokers to wean themselves off much more dangerous tobacco products. On the other, there are those who say that vaping’s risks are unclear and might still rival that of traditional smoking. A study in mice, published today in Proceedings of the National Academy of Sciences, is sure to provide fodder for both sides.

Researchers at New York University exposed mice to e-cigarette vapor (actually heat-generated aerosol that contains nicotine along with some organic solvents) and compared them to a control group just given filtered air. The vaped-upon mice had higher signs of DNA damage throughout several of their organs, namely the lung, heart, and bladder. Their organs were likewise less able to repair DNA. Another experiment found the exact same pattern in human lung and bladder cells exposed to nicotine and a chemical it gets broken down into, nicotine-derived nitrosamine ketone.

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“Based on these results, we propose that [e-cigarette vapor] is carcinogenic and that E-cig smokers have a higher risk than nonsmokers to develop lung and bladder cancer and heart diseases,” they wrote.

As the authors admit, though, tobacco smoke is chock full of other nasty chemicals that can cause cancer along with other health problems, like emphysema. The study seems to reaffirm the basic premise that e-cigarettes aren’t as dangerous as tobacco products, but it isn’t the first to suggest they (not to mention the various flavors added to them) have their own set of risks.

The problem comes when trying to gauge how much of an added risk e-cigarettes pose to your health. It can take years, if not decades of observational research to pin down how carcinogenic something can be. That might be even harder to do for e-cigarettes, since many users are either former or current smokers, and there’d be no easy way to tell where the cancer risk from smoking ends and the cancer risk from vaping begins.

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For people who use vaping to wean themselves off smoking, that uncertainty might be a perfectly fine thing to live with—a smaller cancer risk is still better than nothing.

But the authors point out that it’s the teens who are fast becoming vaping fans, oftentimes because they think there’s no danger involved (Researchers on both sides of the debate have been less divided about the need to regulate e-cigs as strongly as conventional cigarettes, in part to keep them out of teen hands). By 2015, 16 percent of high schoolers said they used e-cigarettes, making it the most popular nicotine product among kids. Most recently, though, that number has mysteriously dropped. And there’s some early evidence that teen e-cigarette users are more likely to make the leap to tobacco smoking as they grow up, while others might become addicted to nicotine when they wouldn’t have otherwise.

Ultimately, whether these trends could lead to a whole new set of problems decades down the road is still a question for which we don’t really have a good answer.